TOPS-20 PASCAL Language Manual - Bitsavers

INPUT AND OUTPUT
7.16 TERMINAL I/O
The PASCAL language requires that the file buffer always contain the
next file component that will be processed by the program. This
requirement can cause problems when the input to the program depends
on the output most recently generated. To alleviate such problems in
the processing of the predeclared text files INPUT and OUTPUT, PASCAL
uses a technique called delayed device access, also know as lazy
lookahead.
As a result of delayed device access, an item of data is not retrieved
from a physical file device and inserted in the file buffer until the
program is ready to process it. The file buffer is filled when the
program makes the next reference to the file. A reference to the file
consists of any use of the file buffer variable, including its
implicit use in the GET, READ, and READLN procedures, or any test for
the status of the fi1e, namely, the EOF and EOLN functions.
The RESET procedure initiates the process of delayed device access.
RESET is done automatically on the prede~lared file INPUT. RESET
expects to fill the file buffer with the first component of the file.
However, because of delayed device access, an item of data is not
supplied from the input device to fill the file buffer until the next
reference to the file.
When writing a program for which the input will be supplied by the
predeclared text file INPUT, you should be aware that delayed device
access occurs. Because RESET initiates delated device access, and
because EOF and EOLN cause the buffer to be filled, you should place
the first prompt for input before any tests for EOF or EOLN. The
information you enter in response to the prompt supplies the data that
is retained by the file device until you make another reference to the
input file.
Example
VAR
I
INTEGER;
BEGIN
WRITE ('Enter an integer or an empty line: ');
WHILE NOT EOLN DO
BEGIN
READLN (I);
WRITELN ('The integer was: ' , 1:1);
WRITE ('Enter an integer or an empty line: ');
END;
WRITELN ('Done');
END.
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